This invention relates to apparatus useful in communication systems, and is particularly directed to a MODEM (modulator-demodulator) useful for the transmission and/or reception of data signals via a high frequency communication link.
Since high frequency radio waves propagate beyond the line of sight by means of ionospheric reflections, long range communication between various locations several hundreds or several thousands of miles apart is possible, without using earth satellite repeaters. Such links are commonly used to and between remote locations, such as off-shore drilling platforms, marine traffic, and airplanes on transcontinental flights. In the past, such systems transmitted voice signals which are of course analog in form, but the need now exists for the transmission of digital signals.
However high frequency radio waves are subject to various forms of deterioration when travelling between a transmission and a receiving point. For example, the signals usally travel over several paths to the same point. Consequently due to different path lengths, a single signal element appears to "smear" in time, or the same signal can arrive time spaced from preceding similar elements, giving the appearance of echo. Due to ionospheric variations, the time shifts often vary, causing a constantly changing signal form. Variation in the ionosphere can also give rise to Doppler shifts in the frequency or phase of the high frequency signal. Further, the signal passing via various signal paths may fade; the phenomenon of fading is both path selective and frequency selective, and is both time varying and narrow-band and wide-band frequency selective, and affects virtually any frequency bands below the maximum usable frequency.
While the above-described problems associated with high frequency communication is significant with respect to analog signals, their effect on a reliable modern data communication link is more profound. Clearly echo and multipath fading can significantly effect the reliability of the data.